1. Field of the Invention
The present invention relates to a process and an apparatus for forming a deposited film by microwave plasma CVD.
2. Related Background Art
Plasma CVD (chemical vapor deposition) process is a process for bringing a specific substance into a plasma state, thereby converting the specific substance into highly active radicals, and bringing the radicals into contact with a substrate, thereby forming a deposited film on the substrate. The plasma CVD process is based on the high activity of radicals and a desired deposited film can be formed on a substrate by properly selecting a radical density and a temperature of the substrate on which the deposited film is to be formed. One of the important points in the CVD process is efficient radical generation.
Generally, glow discharge has been employed for making plasma from a raw material gas for forming a deposited film, and a high frequency wave of 13.56 MHz has been usually used to generate glow discharge. Recently, high density plasma was efficiently generated with a microwave of 2.45 GHz and thus radicals can be efficiently generated. It has been found that it is possible to heat the substrate at the same time. Thus, the plasma CVD process using a microwave has been regarded as promising, and several processes and apparatuses for forming a deposited film by microwave plasma CVD have been proposed.
For example, various processes and apparatuses for forming insulating deposited films of amorphous silicon, which will be hereinafter referred to as "a-Si", or polycrystalline silicon, will be hereinafter referred to as "p-Si", or SiO.sub.2 or SiN or the like as device members for use in semiconductor devices, electrophotographic photosensitive members, image input sensors, pickup devices, photovoltaic elements, or other electronic devices, and optical devices, etc. by microwave plasma CVD have been proposed. One example of the apparatuses for forming a deposited film by microwave plasma CVD is disclosed in JP-A-60-186849. The substrates disclosed by it are so arranged as to surround a microwave energy-introducing zone, thereby forming an internal chamber (that is, a discharge space) and enhancing the utilization efficiency of raw material gas.
The structure of such a conventional apparatus for forming a deposited film by microwave plasma CVD is shown in FIGS. 4 and 5. The apparatus for forming a deposited film is directed to production of cylindrical electrophotographic photosensitive members. FIG. 4 is schematically a vertical cross-sectional view of the conventional apparatus and FIG. 5 is schematically a horizontal cross-sectional view of the apparatus along line X--X of FIG. 4.
A reactor vessel 201, which can be made vacuum and gas-tight, is a substantially cylindrical vessel. Evacuating pipe 204 formed on the side wall of the vessel, the evacuating pipe 204 is communicated with a vacuum pump (not shown in the drawings). Waveguides 203 are fixed substantially at the centers of the upper and lower sides, respectively, of the reactor vessel 201. The waveguides 203 are connected to a microwave power source (not shown in the drawings) through a stub tuner (not shown in the drawings) or an isolator (not shown in the drawings). Dielectric windows 202 for introducing a microwave are provided at the ends of the waveguides 203, respectively, and the vessel 201 is gas-tightly sealed. The dielectric windows 202 are each made from a material capable of efficiently transmitting microwaves from the waveguides 203 into the reactor vessel 201 and capable of making the inside of the reactor vessel 201 vacuum and gas-tight, for example, quartz glass or alumina ceramics. A plurality of cylindrical substrates 205, on which a deposited film is to be formed, are so arranged in parallel to each other as to surround the center zone of the reactor vessel 201. Each of the substrates 205 is supported by a rotary shaft 208 extended upwardly from the lower side of the reactor vessel 201 and heated by a heater 207 inserted into the substrate 205 and likewise extended upwardly therein from the lower side of the reactor vessel 201. The rotary shaft 208 is rotatable without interrupting the vacuum state of the reactor vessel 201, and the other end of the rotary shaft 208 is connected to a driving motor 209 through a reduction gear 210. Thus, the substrate 205 rotates around its center axis in the longitudinal direction (bus direction) by driving the motor 209.
In the reactor vessel 201, a zone surrounded by the cylindrical substrates 205 and the dielectric windows 202 constitutes a discharge space 206, which is substantially a columnar space whose both ends are confined by each of the dielectric windows 202.
A conventional deposited film formation is carried out in the following manner. At first, the reactor vessel 201 is evacuated by a vacuum pump (not shown in the drawings) through an evacuation pipe 204 to reduce the internal pressure of the reactor vessel 201 to 1.times.10.sup.-6 Torr or less. Then, all the substrates 205 are heated to a suitable temperature for film deposition by heaters 207. Raw material gases for film formation are introduced into the reactor vessel 201 through raw material gas introducing pipes (not shown in the drawings). In case of forming an a-Si film, raw material gases such as a silane (SiH.sub.4) gas, a hydrogen gas, etc. are introduced into-the reactor vessel 201. At the same time and in parallel to the introduction of raw material gases, a microwave having a frequency of 500 MHz or more, preferably 2.45 GHz, is generated from a microwave power source (not shown in the drawings) and introduced into the reactor vessel 201 through the waveguides 203 and the dielectric windows 202. As a result, glow discharge is generated in the discharge space 206, and the raw material gases are excited and disassociated by the energy of the microwave to form a deposited film on the surfaces of the cylindrical substrates 205. The substrates 205 are made to rotate by driving the motors 209, and each deposited film can be formed each on the entire peripheral surfaces of substrates 205.
In the case of forming a deposited film by use of the above-mentioned conventional apparatus, when the internal pressure of the reactor vessel is 5 m Torr or less during the film formation, it is possible to obtain a deposited film with practical film thickness and film characteristics uniformly at a high deposition speed, even if a deposited film has a relatively large area, for example, for use of an electrophotographic photosensitive member. However, since only a pair of dielectric windows for introducing the microwave is provided at both ends of the discharge space in the above-mentioned apparatus, the intensity of generated plasma is gradually attenuated as the plasma goes farther from the dielectric windows, when the internal pressure exceeds 5 m Torr during the film formation, and as a result the following problems have been encountered.
Since the intensity of plasma becomes uneven in the discharge space between a pair of the dielectric windows, (1) uneven concentration of radicals formed in the plasma to contribute to the film deposition makes the thickness of a film deposited each on the substrates uneven, (2) uneven distribution of generated radical species makes distribution of deposited film quality uneven, (3) uneven heating of substrates by plasma makes distribution of deposited film quality uneven, and (4) spatially uneven decomposition degree of a doping gas such as diborane (B.sub.2 H.sub.6) or phosphino (PH.sub.3) makes electrical characteristics of a deposited film uneven.